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5. Free Sulphuric Acid.A solution of 20 grams in water is titrated
with N/2-sodium hydroxide in presence of methyl orange.


* *

Pure ammonium sulphate contains 21-21% N and should leave no residue on
ignition. The commercial salt should contain at least 19% N, but usually con-
tains 20-21%,

SODIUM,   NITRATE (Chili Saltpetre)

Minute grey or yellowish crystals containing, as impurities, chlorides,
sulphates, insoluble substances and perchlorates. The more important
tests and determinations are as follows :

1.  Moisture.'5 grams are heated in an oven at no -120" to constant
weight.    The nitrate may also be weighed in a crucible, heated carefully
to incipient fusion, allowed to cool in a desiccator and re-weighed.

2.  Various Impurities.-.....The insoluble matter, chlorides, sulphates,

lime, etc., are detected and, where necessary, estimated by the ordinary
analytical methods.

3.  Detection of Perchlorates.---10 20 grams are dissolved in as much
Welter and filtered, a few drops of the lilt rale being treated on a microscope
slide with I 2 crystals of rubidium chloride and the liquid coloured pink
with one or two drops of dilute potassium permanganate solution.    After
careful evaporation over a, very small flame until a. crust forms at Hie edges of
the liquid, a cover-slip is placed on the drop and the latter examined under
the microscope :   the presence of perehlurate is indicated by dark violet-
red, rhombic, crystals of rubidium perehlorate, often in stellate groupings,
by the side1 of the colourless sodium nitrate crystals.

4.  Determination   of  Perchlonite.-- This   may   be   earned   out    as
follows :

The chlorine in the nitrate is lii'sl determined by one <!' the ordinary
methods. Another portion of 5 or 10 grams of the finely powdered nitrate,
is mixed with pure calcium oxide (H grams) or carbonate (15 giams) (quite
free from chlorine) and the mixture heated lor about 15 minutes in a, pluti
num or porcelain crucible. The mass is then dissolved in dilute nitric
acid and the chlorine again determined. The increase in the percentage
of chlorine, multiplied by tV.|35o, gives .he percentage of Na< 1O4.

If, besides perchlorate, the nitre contains also chlorate,  the latter is eal
ciliated as perchlora.te.

5.  Nitrogen. Sw General Methods, .}, /.'.

6.  Determination of the Sodium Nitrate.-- The amount   of NaNO.,
may be calculated by multiplying the percentage of nitrogen by (ro'ji^,
Usually, however, and especially with nitre lor industrial purposes, thu
nitrate is determined either by decomposing it with sulphuric acid in presence
of mercury and measuring the volume of nitric oxide tunned, or by difference
after the extraneous substances have been estimated.    The methods to
be applied in the two eases are as follows;

i. NiTKciMKTRic MiiTiioi) (Lung*'). This makes use of the Nitrometer,
shown in l-'ig. 4, and consisting of two glass tubes, .1 and />', the formermade up to volume and mixed. An aliquot: part of the liquid is then treated with